Conventionally, coating compositions, inks and like coloring liquids are clear varnishes containing pigment pastes. Pigment pastes are generally prepared by the steps of mixing pigments, resins, organic solvents, and like raw materials in an agitator to prepare a mill base, and then passing this mill base a few times through a bead mill dispersion apparatus or like continuous dispersion apparatus to disperse the pigment.
Specifically, the commonly employed pigment dispersion method comprises the steps of feeding an unprocessed pigment paste stored in a feeding vessel to a dispersion apparatus, temporarily storing the pigment paste obtained by dispersing it in the dispersion apparatus in a receiving vessel, returning the pigment paste stored in the receiving vessel to the dispersion apparatus to redisperse it after the completion of the first pigment dispersion process, and returning the pigment paste which has been subjected to the second pigment dispersion process to the feeding vessel to store it, and then repeating these processes a few times. The above-mentioned manufacturing process, however, disadvantageously requires two vessels, i.e., feeding vessel and receiving vessel, and operations to switch between these vessels.
To overcome these disadvantages, a known technique connects an agitator and a dispersion apparatus via a circulation line to circulate pigment paste between the apparatuses, unifying the feeding vessel and receiving vessel (for example, refer to Japanese Unexamined Patent Publication Nos. 1996-266880 and 2002-306940).
A known bead mill apparatus (cf. Japanese Unexamined Patent Publication No. 1996-266880, Japanese Examined Patent Publication No. 1994-28745 and Japanese Unexamined Patent Publication No. 2002-204969) having a mechanism which separates pigment paste from a grinding medium by the action of centrifugal force caused by the rotation of a rotor has such advantages that it has a large throughput (flow rate); it requires only one vessel because it allows circulation dispersion; and it does not require a switching operation between a feeding vessel and a receiving vessel because it has only one vessel.
However, even if a pigment is dispersed and mixed by using the above-mentioned bead mill apparatus, there is the disadvantage that insufficient agitating and mixing in an agitator may cause mill base to short-path when the pigment flows in and out around the agitator (for example, anchor type, propeller type), and that the efficiency of the pigment dispersion is lowered if there is any pooling in the vessel. Here, “short-path” means that fluid supplied in an agitator is discharged from the agitator without fully being agitated.
Accordingly, to efficiently perform agitating and mixing in the agitator, a double-shafted mixer having a high-speed agitator and a low-speed anchor type agitating blade which removes the pooled mill base off the vessel wall was developed.
However, said double-shaft mixer has the problem of high installation cost. In addition, since a small interval between the vessel wall and anchor type agitating blade makes cleaning the mill base by injecting a cleaning solvent difficult, the mixer still has a problem in its ability to be cleaned when the mixer is applied to the production of coating compositions, which requires the frequent replacement of materials.
There are other known mixers, for example, single shaft mixers, than the above-mentioned agitators (for example, refer to Japanese Patent No. 3224498 and Japanese Examined Patent Publication No. 1989-37173). Although these agitators are suitable for simply mixing a fluid in a vessel homogenously, when they are used for circulation dispersion systems which drive fluid drawn from a lower part of the vessel from a return pipe provided in an upper part of the vessel into the vessel, and return the drawn fluid to the return pipe through a dispersion apparatus, they have the following disadvantage: as the circulating flow of the fluid in the vessel becomes faster, the fluid provided by the return pipe fails to be mixed and short-paths in the vessel because it is instantaneously drawn from the lower part of the vessel. Furthermore, it is less effective than an anchor type agitating blade in drawing fluid off the inner wall portion of the vessel in the agitator, mixing it, and circulating it. Therefore, pigment paste with high structural viscosity is likely to pool on the wall of the vessel and thus is disadvantageously difficult to mix and agitate.
To overcome the aforementioned problems, the inventors of the present invention have previously improved the constitution of paddle blades and proposed an agitator which can be applied to a circulatory system with a large flow rate, can deal with a variety of fluids, changes in fluid volume, and has an excellent ability to mix and disperse fluids with different viscosities ranging from low to high and cleanability (refer to Japanese Patent No. 3189047).
Moreover, the production of coating compositions and like coloring liquids is often in small batches of a wide variety of products. Therefore, every time the color is changed, the agitating vessel and other portions which come in contact with the pigment paste need to be cleaned. In a known cleaning step, for example, a cleaning device ejects a cleaning liquid from a cleaning nozzle connected to a cleaning liquid tank into the agitating vessel (for example, refer to Japanese Patent No. 3189047). This cleaning device showers the inner wall of the agitating vessel and the surface of the agitating blade with the cleaning liquid from the cleaning liquid tank via the cleaning nozzle to wash away pigment paste deposited therein. The cleaning liquid ejected from the cleaning liquid nozzle into the agitating vessel is immediately drawn out from the bottom of the agitating vessel, collected and recycled.